Progressive multifocal leukoencephalopathy (PML) is a rare and potentially fatal opportunistic infection of the central nervous system that is caused by a ubiquitous polyomavirus, the JC virus (JCV). While JCV is present at very high rates in the general population, PML remains a rare disorder, albeit an important one because of the poor survival and the severe neurological sequelae, and the recently demonstrated association with a variety of useful therapies, for example, natalizumab in multiple sclerosis (MS). A number of risk factors for PML have been described but these are better viewed as necessary but not sufficient. While these risk factors are highly relevant, they do not, on their own, predict who will develop PML, since the vast majority of individuals with these risk factors will not develop the disorder. Other factors need to be considered and there is growing evidence for the role of host genetic factors in susceptibility to PML.
The ability to more accurately predict who is at risk of developing PML will be of enormous benefit in the context of drug treatment with compounds that are highly effective in their disease context (natalizumab in MS, for example) but carry a small risk of a devastating disorder. There is a need to develop a companion diagnostic testing, in order to effectively exclude those that were at risk of PML, in the process reassuring those with negative tests about their dramatically reduced risk of developing PML.